You can say to a child, "a watershed is the area drained by a river system," and be technically correct, but to a child raised in an urbanenvironment your words have little meaning. It may not be feasible to organize an outdoor field trip, and given the scale of landforms involved, not that helpful. This is where models come in handy. Landform models in clay, dirt, or sand can show how water drains an area, but to draw in your students, get them interested, there's nothing like the tactile experience of feeling water trickle into your palms. Gather a group of students. Have then cup their palms, creating valleys. Then place their hands next to each other, so that their thumbs and index fingers form not only ridgetops, but watershed boundaries.To simulate a rainshower moving across these different watersheds, use a slow and deliberate sprinkling of water. (Pouring a deluge across their hands might be fun, but not as helpful.)

For a school assembly, where a question of scale requires a larger demonstration, I've found the following technique helpful:

If you're on a stage: have the volunteers sit cross-legged, facing the audience. If you are at the same level as the audience, keep them standing.

Place the tarp over the volunteers' heads, and gather the tarp to create a mountain fold between the two heads. If they're sitting, your volunteers legs will form natural depressions. If they are standing, you may need them to hold their arms out in front of them to support the tarp and create valleys.

Explain to the audience that these volunteers each represent a different watershed. If you're familiar with the local watershed, you can indicate specific towns or places. ("Susan, here, is going to be San Jose, and Michael, here is going to be Santa Cruz. They're separated here by the Santa Cruz mountains.")

Now you can demonstrate how each watershed boundaries delineate differing drainage areas by pouring water onto the tarp, on both sides of the fold (the watershed boundary), narrowly missing your volunteers' heads.

(optional) To increase your audience's enjoyment, you can good-naturedly pour the water directly onto your volunteers heads. Advanced performers: good-naturedly threaten to douse them, then say you were "just kidding," back off, and then while seemingly distracted, "accidentally" douse your volunteers. Look in mockhorror at what you've done.

Sum up what you've just demonstrated:

"Okay, so what did we learn? We learned to never trust an assembly presenter. Er, no. We saw the water poured on Susan in San Jose drained to one area, and the water poured on Michael in Santa Cruz drained to a different area. San Jose and Santa Cruz have different watersheds, separated by these mountains."

A watershed is a marvelous thing to consider: this process of rain falling, streams flowing, and oceans evaporating causes every molecule of water on earth to make the complete trip once every two million years. The surface is carved into watersheds-- a kind of familiar branching, a chart of relationship, and a definition of place. The watershed is a first and last nation whose boundaries, though subtly shifting, are unarguable. Races of birds, subspecies of trees, and types of hats or rain gear often go by the watershed. For the watershed, cities and dams are ephemeral and of no more account than a boulder that falls in the river or a landslide that temporarily alters the channel. The water will always be there, and it will always find its way down. As constrained and polluted as the Los Angeles River is at the moment, it can also be said that in the larger picture that river is alive and well under the city streets, running in giant culverts. It may be amused by such diversions. But we who live in terms of centuries rather than millions of years must hold the watershed and its communities together, so our children might enjoy the clear water and fresh life of this landscape we have chosen. From the tiniest rivulet at the crest of a ridge to the main trunk of a river approaching the lowlands, the river is all one place and all one land.

As well as the very literal meaning of watershed as a 'line of division between two adjacent rivers or lakes' (Webster 1913), there is a figurative meaning which is used far more frequently in the UK than in North America.

In this figurative meaning, watershed is used to refer to any critical or defining division, balance, or moment, such as crucial piece of legislation, or a vitally important strategic battle in a war.

However, its most common usage, and almost certainly the one that has maintained its currency as a figure of speech, is in connection with television. In the UK, terrestrial (non-cable/satellite) broadcasters refer to 9pm simply as 'the watershed', after which time it is permissibletobroadcastprettymuchanything.